1. Field of the Invention
This invention relates to a method and apparatus for detecting a position of a center point of a circular pattern, such as a pupil pattern, from a digital photographic image. This invention also relates to a computer program for causing a computer to execute the method of detecting a position of a center point of a circular pattern.
2. Description of the Related Art
Operations for detecting positions of center points of circular patterns from digital photographic images have heretofore been performed in various fields, such as detection of positions of center points of circular patterns of circular mechanical parts and detection of positions of center points of pupil patterns in photographic images of faces. Heretofore, various techniques, such as template matching techniques and Hough transform techniques, have been utilized for detecting circular patterns from digital photographic images of objects. Of the techniques described above, the Hough transform techniques have been utilized as techniques capable of detecting incomplete circular patterns, such as circular patterns having breakages due to noise, and pupil patterns in which only partial areas of pupils are illustrated due to eye shutting. (The Hough transform techniques are described in, for example, Japanese Unexamined Patent Publication No. 2003-70742.) Specifically, with the Hough transform techniques described above, a digital photographic image is subjected to binarization processing, and an edge is detected from a binary image having been obtained from the binarization processing. Also, by use of data on pixels corresponding to the detected edge, voting is performed on a Hough space corresponding to three parameters (i.e., an X coordinate of a center point of a circle, a Y coordinate of the center point of the circle, and a radius r) for the detection of an annulus. Thereafter, coordinates, which have received the largest number of votes as a result of the voting, are acquired, and values of the three parameters for annuluses are calculated in accordance with the thus acquired coordinates. Further, the annulus center coordinate values (X, Y) having thus been calculated are taken as the coordinate values of the center point of the circular pattern to be detected. In this manner, the position of the center point of the circular pattern is detected.
The aforesaid techniques for detecting the position of the center point of the circular pattern with the Hough transform have large effects of detecting the circular patterns having breakages and the positions of the center points of the circular patterns having breakages. However, with the aforesaid techniques for detecting the position of the center point of the circular pattern with the Hough transform, only the data on the pixels corresponding to edges in the digital photographic images are utilized. Therefore, the aforesaid techniques for detecting the position of the center point of the circular pattern with the Hough transform have the problems in that the aforesaid techniques are apt to be adversely affected by noise.